An optical wavelength division multiplexing technology has been maturely applied in the field of submarine fiber optical communication for many years. In some wavelength division submarine cable systems, a branch is provided. For example, the branch is connected to a branch station by using a BU (branching unit). Inside the BU, one or more OADMs (Optical Add-Drop multiplexer) may be included to constitute an optical add-drop multiplexing branching unit (OADM BU). The BU is may also be externally connected to one or more OADMs to implement functions of an OADM BU. In both manners, a part of signals in trunk signals are dropped to a branch station by using an OADM and a new signal is added to the trunk signals from the branch by using the OADM.
For an application environment of an optical add-drop multiplexer (OADM), refer to FIG. 1. In FIG. 1 (in a case of an OADM BU), Stations A, B, and C are three different stations. The Stations A and B are trunk stations, and a volume of signal transmission is large. The Station C is a branch station, and a volume of signal transmission between the Station C and the trunk stations A and B is relatively small. A branch station drops a signal and uploads a signal by using an OADM BU to implement communication with the stations A and B. The RPT (Repeater) in the figure is an optical amplifier of a submarine cable line, Add and Drop on the branch station C respectively represent ports of an add signal and a drop signal. All arrow directions in FIG. 1 are transmission directions of optical signals.
As shown in FIG. 2, FIG. 2 is a schematic diagram of transmission of a signal when it passes through an optical add-drop multiplexer in an OADM BU in FIG. 1. Transmission signals are classified into four types, which respectively are: an Express (express) signal, which is represented by a rectangle filled with horizontal lines, and bears communication from a trunk station to another trunk station; a Drop signal, which is represented by a rectangle filled with oblique lines from the upper right to the lower left, and bears communication from a trunk to a branch; an Add signal, which is represented by a rectangle filled with diagonal squares, and bears communications from a branch to a trunk; and a Loading signal, which is represented by a black rectangle, where the Loading signal is a wave loading signal in a branch, the loading signal is a signal that is used to perform power balancing and bears no service, and power of the loading signal is the same as power of an express signal. Generally, a wavelength of a drop signal is the same as a wavelength of an Add signal. Squares marked with “Input” and “Output” in FIG. 2 are respectively a port through which a trunk signal enters an OADM, and a port through which a trunk signal is output from the OADM.
In FIG. 2, trunk input signals pass through a Splitter and are split into two lines of signals: one line is dropped to a branch, where a branch station selects a drop signal to receive; and the other line passes through a band block filter (BBF), where a drop signal is blocked, and then an express signal is combined with a branch Add signal and a combined signal is output. When a repeater is used in the branch, in order to balance input power of the repeater, a Loading signal is also input together with the branch Add signal. The Loading signal is filtered out by a band pass filter (BPF) in a transmission direction of the add signal, and the Add signal that passes through the band pass filter and the express signal are combined by using a coupler and then output. FIG. 2 shows signals in one transmission direction in an OADM BU. There is also another signal transmission direction that is completely symmetrical to the one transmission direction, and details are not described herein again.
Referring to FIG. 2, express signals may be dropped to a branch, and consequently, an express signal communicated between trunk stations may be intercepted from a branch station. Therefore, eavesdropping cannot be prevented. In order to prevent eavesdropping, a commonly used solution is a solution based on a transmission protocol, where an OTU (optical transponder unit) board adds a security password to an overhead byte in a service frame structure of a to-be-transmitted signal, different security passwords are set for different target stations, and payload data in a frame structure may be read if a security password read by an OTU board of a target station is the same as a security password preset by the OTU board. In this way, an OADM branch station can receive only data of a drop signal.
However, when the solution based on a transmission protocol is used, if the password is cracked or leaked, content of a trunk signal may still be eavesdropped from a branch. Therefore, information security for anti-eavesdropping is not high.